1. Field of the Invention
The present invention relates to a radiation-sensitive composition and, more particularly, a radiation-sensitive composition which includes a nonpolymeric silsesquioxane and a polymer.
2. Description of the Related Art
In the fabrication of integrated circuits, lithography is used to generate pattern structures on the semiconductor and various materials for the construction of the desired circuit structures. A continuing demand in view of the ever increasing desire in the semiconductor industry for higher circuit density in microelectronic devices has prompted lithographic engineers to develop improved lithographic process.
The most direct method of achieving higher area density is to improve the resolution of circuit patterns in resist films. One way of improving the resolution in resist is to migrate to shorter wavelength from 365 nm to 248 nm, then to 193 and 157 nm, or to adopt non optical system such as EUV, E-beam and X-ray. The other way is to increase the numerical aperture (NA) of the lens system of the lithographic imaging tool at a given wavelength.
However, increasing the NA results in a decrease in the depth of focus (DOF) of the imaging radiation, thereby requiring a reduction in the thickness of the imaging resist film. Another reason for requiring thinner resist is that the resist images usually collapse during the developing step with aqueous base if the aspect ratio is greater than 3. However, a thin resist may not have sufficient etch masking capability.
One approach that enables the use of higher NA exposure tools as well as a thinner photoresist film is multilayer resist processing such as bilyer and trilayer resists. Bilayer resists generally comprise a top thin film imaging layer coated on a thick organic underlayer. The resist is patterned by (i) imagewise exposure and development of the top layer, and then (ii) anisotropically transferring the developed pattern in the top layer through the thick underlayer to the substrate.
Suitably, the top layer contains precursors to etch resistant oxides such as silicon, boron, germanium, or metals such as titanium, iron, tin, etc., which enable the use of oxygen-reactive ion etching (RIE) in the image transfer step. Silicon-containing resists are the most commonly used bilayer top imaging resists.
Two major type of bilayer resists are known in the art. One uses silsesquioxane polymer (SSQ polymer) as back bone, and the other uses silicon elements in the side chain. Short wavelength and high energy radiation source such as 193 nm, 157 nm and EUV cleaves bonds on the side chain causing a Si component to outgas from resist during exposure and deposit on the lens. The organic component deposited on the lens can be cleaned but not the Si component. Therefore, it is not desired to use polymers with Si components on the side chain. On the other hand, it is hard to synthesize SSQ polymer with consistent structure and molecular weight due to its various extent of condensation during polymerization.
Polyhedral oligosilsesquioxanes (POSS) has a well defined structure and no breakable side chain Si components. For example, U.S. Pat. No. 6,420,084, inter alia, discloses the linkage of the POSS unit on the side chain of a polymer in a chemically amplified resists system. U.S. application Pub. No. 20020182541 also discloses a polymer containing POSS on the side chain for a chemically amplified resist system. This patent application publication also discloses using nonactive POSS (i.e. no acid labile groups attached to it), as an additive to photoresist to improve etch properties. The additive approach has the advantage of being simple and easy in formulating resists.
However, small silicon compounds have outgassing concern, while non-active POSS is a big molecule with difficulty in obtaining proper dissolution properties for resist to perfom well especially in a positive tone resist system. In the negative tone resist, the resist requires aqueous soluble properties. Most commercial POSS materials have cyclopentyl groups linked to the corner, which has no solubility in aqueous base and are only suitable for resists used in solvent developable systems.